Retroviral-mediated transfer of normal human Adenosine Deaminase Deficiency (ADA) cDNA into hematopoietic stem cells of patients with ADA- deficient SCID may result in the production of mature T lymphocytes expressing ADA enzyme activity, with restoration of immune function. ADA gene transduction of hematopoietic stem cells will be performed from patients with ADA deficiency and the transduced cells will be transplanted into the patients. We will then deterimine the efficacy of ADA gene tansfer and the expression by serial examination of peripheral blood lymphocytes and hematopoietic cells. We will quantitate the frequency of cells which contain the ADA cDNA by semi-quantitative DNA-PCR and the level of expression from the vector by RT-lined PCR and ADA enzyme assay. Additonally, the effects of ADA gene expression on the immune function of the transplanted patients will be examined. In all, these clinical studies will determine the effectiveness of current retroviral-mediated gene transfer techniques to introduce genes into human hematopoietic stem cells, which may be used to treat a number of genetic diseases and AIDS. Additionally, the specific benefits for the immune deficiency of ADA- deficient SCID will be assessed.